This invention relates to a method of controlling the air-fuel ratio in an internal combustion engine of an automobile or the like provided with an O.sub.2 sensor at both the upstream and downstream sides of a catalyst converter for purifying the exhaust gas.
In an internal combustion engine so arranged as to supply fuel to the intake passage of the engine through an injector controlled by an electronic control device, the amount of fuel supplied in each cycle of the engine is generally determined in the following manner. The injector is provided with a solenoid valve. When a fuel injection signal (pulse voltage) is applied to the electromagnetic coil of the valve by the electronic control device, fuel is injected through a nozzle in an amount corresponding to the time width of the applied signal. The electronic control device determines first a basic injection time period in accordance with the intake air pressure, the engine speed, or the intake air amount detected by an air flowmeter or the like device, and then corrects the basic injection time period by various correction coefficients determined in accordance with the engine conditions, thereby to determine a current injection time period. The control device then applies a pulse signal of a time width corresponding to the current fuel injection time period to the fuel injector at a predetermined injection time, thereby to supply a proper amount of fuel to the engine.
The three-way catalyst of a catalyst converter provided in the exhaust system of an engine can most effectively oxidize and reduce CO, HC and NOx contained in the exhaust gas, if the air-fuel ratio of the mixture gas is maintained within a narrow range about the stoichiometric air-fuel ratio. The stoichiometric air-fuel ratio means the air-fuel ratio of a mixture gas which contains the exact amount of oxygen required to make the fuel contained in the mixture gas combust perfectly. In the case of gasoline the stoichiometric air-fuel ratio is usually between about 14.4 and 15. A mixture gas or the air-fuel ratio thereof is said to be "lean" if the ratio is higher than the stoichiometric value, and "rich" if it is lower than the stoichiometric value.
An O.sub.2 sensor for detecting the oxygen concentration of the exhaust gas is provided near the inlet port of the catalyst converter. The O.sub.2 sensor is so designed as to generate a low voltage when the air-fuel ratio of the mixture gas is lean and the oxygen concentration of the exhaust gas is high, and a high voltage when the air-fuel ratio of the mixture gas is rich and the oxygen concentration of the exhaust gas is low. The O.sub.2 sensor has a characteristic that its output voltage changes drastically about the stoichiometric air-fuel ratio.
In an engine provided with the above-mentioned device, the air-fuel ratio of the mixture gas is adjusted to about the stoichiometric value in the following manner. The electronic control device has an air-fuel ratio feedback correction coefficient set therein beforehand. The basic injection time period is multiplied by the coefficient, which is changed about 1 (one) in accordance with the output voltage of the O.sub.2 sensor. When the electronic control device has detected by the output voltage of the O.sub.2 sensor a change of the air-fuel ratio of the mixture gas from the lean side of the stoichiometric ratio to the rich side thereof, the device reduces the air-fuel ratio feedback correction coefficient skippingly by a predetermined relatively large amount at first and then gradually by a minute amount. As a result, since the fuel injection amount decreases, the air-fuel ratio of the mixture gas changes from the rich side toward the stoichiometric ratio. When the electronic control device detects a change of the air-fuel ratio of the mixture gas from the rich side to the lean side by the output voltage of the O.sub.2 sensor, the device increases the air-fuel ratio feedback correction coefficient skippingly by a relatively large amount at first and then gradually by a minute amount. As a result, since the fuel injection amount increases, the air-fuel ratio of the mixture gas changes from the lean side to the stoichiometric value. By repeating the feedback control operation the air-fuel ratio of the mixture gas approaches the stoichiometric level thereby to effectively purify the exhaust gas by the three-way catalyst.
With a single O.sub.2 sensor, however, a required air-fuel ratio feedback control may not be effected because of adverse influences by deterioration of the ability of the O.sub.2 sensor caused by aging or differences in the output characteristics between different O.sub.2 sensors or differences in the fuel injection amount between the injectors of different cylinders of the engine, so that the air-fuel ratio is likely to be deviated from near the stoichiometric value. If the catalyst converter is connected to the concentration portion of an exhaust manifold in order to make the three-way catalyst attain an activating temperature as soon as possible so as to purify the exhaust gas, then the O.sub.2 sensor is disposed at the concentration portion of the exhaust mainfold. In this arrangement the output voltage of the O.sub.2 sensor may be influenced by the exhaust gas from a particular one of the cylinders, or deterioration of the O.sub.2 sensor may be accelerated by hot exhaust gas.
To overcome the above defects, it has been proposed in the prior art to provide a second O.sub.2 sensor at the downstream side of the catalyst converter in addition to the above-mentioned first O.sub.2 sensor, so that the feedback control of the air-fuel ratio may be conducted by the output voltage of both O.sub.2 sensors (See U.S. Pat. Nos. 4,251,989 and 4,712,373). In the proposed arrangement of the prior art, the electronic control device has set therein a feedback control value which is changed in accordance with the output voltage of the second O.sub.2 sensor, and the above-mentioned basic injection time period is directly or indirectly corrected by the feedback control value. The fuel injection amount increases as the feedback control value increases and decreases as the value decreases. At the downstream side of the catalyst converter, the exhaust gas from all the cylinders of the engine is in a well-stirred condition, and the oxygen concentration of the exhaust gas is substantially in an equilibrium state by the function of the three-way catalyst. As a result, the output voltage of the second O.sub.2 sensor changes more slowly than that of the first O.sub.2 sensor. In particular, in case the mixture gas adjusted on the basis of the output voltage of the first O.sub.2 sensor is totally at the rich side, the output voltage of the second O.sub.2 sensor remains rich for a long time. On the contrary, in case the total mixture gas is at the lean side, the output voltage of the second O.sub.2 sensor remains lean for a long time. As mentioned above, while feedback control of the air-fuel ratio is being conducted by using the signal from the first O.sub.2 sensor, the air-fuel ratio of the mixture gas as a whole is detected by the signal from the second O.sub.2 sensor, and in accordance with the result of the detection the feedback control value is changed by a predetermined value at every predetermined gating time. In this manner the air-fuel ratio of the mixture gas can be accurately adjusted approximately to the stoichiometric value, and the exhaust gas can be effectively purified by the three-way catalyst.
In the above-mentioned control, however, the time interval at which the air-fuel ratio is controlled by the output voltage of the second O.sub.2 sensor is several times longer than the time interval at which the air-fuel ratio is controlled by the output voltage of the first O.sub.2 sensor. Therefore, if the above-mentioned feedback control value is gradually increased or decreased by a predetermined value when the air-fuel ratio of the mixture gas has changed from the rich to the lean side or from the lean to the rich side, the fuel supply amount can not be adjusted quickly. This poses a problem that the time period during which the total mixture gas is rich or lean can not be shortened, with resulting difficulty in improving the efficiency of purification of the exhaust gas.